Gravitropism of hypocotyls of wild-type and starch-deficient Arabidopsis seedlings in spaceflight studies

Abstract

The major purpose of this spaceflight project was to investigate the starch-statolith hypothesis for gravity perception, and a secondary goal was to study plant growth and development under spaceflight conditions. This research was based on our ground studies of gravity perception in the wild type and three starch-deficient (one starchless and two reduced starch) mutants of Arabidopsis thaliana (L.) Heynh. Dark-grown seedlings that developed in microgravity were given one of several (30 min, 60 min, or 90 min) 1-g stimuli by an on-board centrifuge, and additional controls for seedling development also were performed. These latter control experiments included a morphological study of plants that developed in space in microgravity (F microg), in space on a centrifuge (F 1g), on the ground (G 1g), and on a rotating clinostat on the ground. Since elevated levels of ethylene were reported in the spacecraft atmosphere, additional controls for morphology and gravitropism with added ethylene also were performed. While exogenous ethylene reduced the absolute magnitude of the response in all four strains of Arabidopsis, this gas did not appear to change the relative graviresponsiveness among the strains. The relative response of hypocotyls of microgravity-grown seedlings to the stimuli provided by the in-flight centrifuge was: wild type > starch-deficient mutants. Although the protoplast pressure model for gravity perception cannot be excluded, these results are consistent with a statolith-based model for perception in plants.

Ethylene Modulates Flavonoid Accumulation and Gravitropic Responses in Roots of Arabidopsis

“…If flavonoid expression is under the same light control in maize seedlings as in Arabidopsis seedlings (Wade et al, 2001;Muller et al, 2005), their dark-grown seedlings may mimic our tt4 mutants with an absence of flavonoid synthesis. This complex role of ethylene, with both stimulatory and inhibitory effects on root gravitropism, may explain the contradictory conclusions in the literature on the question of whether ethylene regulates gravitropism (Wheeler and Salisbury, 1981;Kaufman et al, 1985;Harrison and Pickard, 1986;Wheeler et al, 1986;Lee et al, 1990;Woltering, 1991;Kiss et al, 1999;Madlung et al, 1999). Because both ethylene and flavonoid synthesis are regulated by light, nutrients, and other environmental signals (WinkelShirley, 2002;Vandenbussche et al, 2003), it is likely that these contradictory results come from studies in which the synthesis of these molecules is differentially regulated.…”

“…Exogenous applications of ethylene gas or the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) were used to test the role of ethylene in gravitropic responses in a range of species, with contradictory results. In some experiments, ethylene treatment clearly reduced gravitropic responses (Wheeler and Salisbury, 1981;Wheeler et al, 1986;Lee et al, 1990;Kiss et al, 1999;Madlung et al, 1999), whereas others showed no effects (Kaufman et al, 1985;Harrison and Pickard, 1986;Woltering, 1991). A recent study indicated that ethylene increased gravitropic curvature in etiolated maize (Zea mays) roots under conditions where ethylene inhibited elongation (Chang et al, 2004).…”

Analyses of a Gravistimulation-Specific Ca2+ Signature in Arabidopsis using Parabolic Flights

“…S4). Long-term (hours to days) morphological responses of plants under hypergravity or mg conditions have been described using plants grown in a centrifuge (Fitzelle and Kiss, 2001;Matsumoto et al, 2010) or aboard the space shuttle/station (Kiss et al, 1999(Kiss et al, , 2012Paul et al, 2012;Roux, 2012), respectively. Such reports indicate that plants can respond with alterations in growth and development to a variety of gravitational accelerations.…”

“…3B), suggesting that these plants could only generate a gravitropic [Ca 2+ ] c when displaced from their vertical orientation by rotation. Yet, previous research has shown that Arabidopsis seedlings, geminated and grown in space, showed little gravitropic growth under continuous mg conditions (Kiss et al, 1999), whereas continuous hypergravity conditions, applied parallel to their growth axis, caused morphological changes (e.g. alterations in hypocotyl length and diameter; Matsumoto et al, 2010).…”

Space, the final frontier: A critical review of recent experiments performed in microgravity

“…In fact, some older facilities, such as the Biorack on the space shuttle, have shown the utility of a 1-g in-flight control to distinguish deleterious environmental effects of spaceflight from microgravity such as the build-up of ethylene gas (Fig. 1) [11]. To account for these secondary (or indirect) spaceflight effects, the use of 1-g in-flight controls can be implemented.…”